Problem: Nadia is 15 years older than Stephanie. Fifteen years ago, Nadia was 4 times as old as Stephanie. How old is Stephanie now?
Solution: We can use the given information to write down two equations that describe the ages of Nadia and Stephanie. Let Nadia's current age be $n$ and Stephanie's current age be $s$ The information in the first sentence can be expressed in the following equation: $n = s + 15$ Fifteen years ago, Nadia was $n - 15$ years old, and Stephanie was $s - 15$ years old. The information in the second sentence can be expressed in the following equation: $n - 15 = 4(s - 15)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $s$ , it might be easiest to use our first equation for $n$ and substitute it into our second equation. Our first equation is: $n = s + 15$ . Substituting this into our second equation, we get the equation: $(s + 15)$ $-$ $15 = 4(s - 15)$ which combines the information about $s$ from both of our original equations. Simplifying both sides of this equation, we get: $s + 0 = 4 s - 60$ Solving for $s$ , we get: $3 s = 60$ $s = 20$.